The long-range purpose of this project is to investigate the role of type C retroviruses in the etiology of lymphoid cell neoplasia and the immunologic responses of the host to tumor-associated antigens, and to characterize the structural and functional markers of various tumor cells. The topics of current interest are: 1) Further characterization of xenotropic viruses isolated from SJL/J strain of mouse. 2) Further studies on a new dualtropic retrovirus from Abelson virus-induced lymphoma in BALB/c mice. 3) Discovery of amphotropic viruses from RBL-5, a Rauscher-MuLV-induced tumor, and YAC, a Moloney-MuLV induced tumor. 4) Study of the role of a B-tropic virus isolated from 11A (a Moloney-MSV transformed BALB/3T3 cell line) in the tumor immunity induced by 11A against Meth A tumor transplantation. 5) Attempts to isolate hybridomas exhibiting natural killer (NK) cell activity, and further studies on NK activity associated with RCN of SJL/J mice. 6) Activation of endogenous retroviruses from cells by treatment with hycanthone. 7) Virologic and immunologic studies of established trophoblast cultures.